This invention relates to a method and apparatus for automatically backwashing a filter. More particularly, this invention relates to a method and apparatus for automatically propelling a charge of backwash fluid by a compressed gas such as steam to backwash a filter in a filter pair. Still more particularly, this invention relates to a method and apparatus for backwashing a filter with at least three backwash phases, a first mainly comprising a charge of backwash fluid passing through the filter with significant speed, a second relating to use of flashing steam for secondary cleaning and scrubbing of the filter elements, and a third relating to use of low pressure steam for autoclaving and sterilizing the filter.
Pressure filtration of fluids through various cylindrical configurations of wound or woven filter media is well known to the art. Precoating of such filter septums with a pulverized solid, forming an aggregate interlocking mass thus providing a filter medium with submicron porosity, was developed at least as early as 1942 by the U.S. Army Corps of Engineers. Generally, an unfiltered fluid is introduced from the peripheral surface to an axial filtered product collection chamber of the filter septum or septums until an accumulation of suspended solids upon the surface sufficiently obstructs the porosity of the media to inhibit fluid passage.
Subsequent cleaning of the barrier media by a liquid counterflow through the media is an equally well-established principle, and is regularly employed wherever cylindrical septums are used as a primary filter media or as the base barrier for precoat filtration. For example, U.S. Pat. No. 3,794,169 to Sisk et al. discloses an apparatus for producing a rapid reduction in housing pressure followed by a high velocity slug of liquid to dissociate collected solids from a filtration membrane surface. Such an apparatus constitutes an apparent improvement in that the slug of high velocity liquid produces transient shear forces between the liquid and one side of the membrane to dissociate the solids and gels which tend to collect adjacent to the one surface of the membrane, thereby improving the rate at which filtrate flows through the membrane.
An improved liquid filter in which contaminated liquid is pumped into an inlet chamber, through a filter element and into an outlet chamber uses compressed air held in compartments in the inlet and outlet chambers. When the filter element becomes clogged, it is isolated by valves, and the compressed air in the inlet chamber is vented to permit the compressed air in the outlet chamber to force liquid back through the filter element to dislodge deposited contaminants. Such a system is disclosed in U.S. Pat. No. 4,282,105 to Crowe. Other systems also perform a backwash of a filter by releasing a stored volume of a gas retrograde to the filter flow through the septums, as shown for example in U.S. Pat. Nos. 3,794,169; 4,089,781; 4,217,700; and 4,552,669, each of which is directed to pneumatic backwash systems.
A severe limitation on the backwash selected is the ability of the filter medium to withstand the flow and velocity required to dislodge entrapped solids and retain its physical integrity. The backwash evolution is therefore a carefully engineered balance of velocity, volume, and density of the backwash fluid to achieve the required degree of cleanliness.
Despite the number of systems available and the understandings of the prior art, no such system is yet currently available which achieves multiple levels of laving with change of state of the laving fluid occurring through the filter medium within the filter. For example, for filters used in the food or beverage industries, it would be preferable to backwash the filter under conditions which achieve a sterilization of the filter as well as a backwashing to dislodge entrained solids. Thus, it has remained a problem in the art to develop an operating system which includes a filter element which backwashes a liquid charge through the filter elements, using superheated water flashing to steam under controlled circumstances to provide a secondary scrubbing action and also performing a germicidal or autoclaving action on the filter elements in a controlled manner.
Accordingly, it is a general object of this invention to provide a novel operating system which includes means for backwashing the filter element with a gas-propelled slug of liquid.
It is another overall object of this invention to provide a novel operating system which includes a filter element wherein the control of the backwashing of the filter element is automatic.
It is another overall object of this invention to provide a pressure vessel in combination with a filter element so that a compressed bubble of gas, such as steam, provides motive force to a volume of liquid, such as water, to propel a charge of the liquid through the filter element.
It is still another overall object of this invention to provide a quick-acting valve in combination with a pressure vessel housing fluid so that a charge of backwash fluid having a volume about equal to the filter volume is propelled through the elements as a solid slug of water and moving with sufficient speed that the thermal wavefront of the expanding steam cannot cause two-phase flow or flashing within the initial volume of fluid reaching the filter elements.
It is still another feature of this invention to provide a secondary cleaning action on the filter elements by causing the thermally superheated volume of water to flash to steam in a controlled manner after an initial volume of superheated water has laved the filter elements.
It is still another feature of the invention to control the expansion of the escaping steam volume so that the pressure is at about 10 to about 15 psig whereupon the filter valves isolate the filter vessel when full of low pressure steam thus permitting the low pressure steam to dissipate its heat as it cools to perform a germicidal or autoclaving action upon the filter elements.
It is still another object of this invention to provide a microprocessor-controlled scheme for sequencing the valves in a multi-filter system to provide a controlled backwash for the filter on an automatic basis.
Finally, it is another object of this invention, for a filter system of the type described, to cause the cooling of the low pressure steam in the laved vessel to contract to a minute fraction of its initial volume upon turning to liquid, thereby providing an advantageous partial pressure or vacuum condition inside the vessel.
These and other objects of the invention will become apparent from a detailed written description of the invention which follows, taken in conjunction with the accompanying drawings.